Search Interpretations


Mr. David H. Bradley

Canadian Trucking Alliance

Butterworth House

324 Somerset St. West, Suite 100

Ottawa, ON Canada K2P 0J9

Dear Mr. Bradley:

This responds to your letter concerning an aerodynamic device manufactured by Advanced Transit Dynamics, Inc. (ATDynamics) called a boat tail or trailer tail that some of your member motor carriers would like to install on their van trailers to reduce the fuel consumption of their vehicles. (In this letter, we will refer to these aerodynamic devices generally as trailer tails and to the device manufactured by ATDynamics as the ATDynamics TrailerTail.) You state that use of trailer tail technology would help meet a California Air Resources Board regulation that requires all U.S. and Canadian 53-foot van trailers to achieve a 5 percent overall reduction in greenhouse gas emissions by 2016.


In your letter, you ask for our acceptance of an October 10, 2008 letter from the Director of the Federal Highway Administrations (FHWAs) Office of Freight Management and Operations to ATDynamics, a copy of which you enclosed. (October 10, 2008 letter from Anthony Furst to ATDynamics Andrew Smith.)

In the letter, Mr. Furst discusses FHWA regulation 23 CFR 658.16, Exclusions from length and width determinations. Subsection (b) of 658.16 sets forth exclusions from either the measured length or width of commercial motor vehicles, as applicable, and lists aerodynamic devices in subpart (4).

Mr. Furst states in the letter that FHWA regulation 23 CFR 658.16(b)(4) excludes an aerodynamic device from the measured length of a commercial motor vehicle provided: (1) the device is not capable of carrying cargo; (2) the device does not extend beyond 5 feet of the rear of the vehicle; (3) the device does not obscure tail lamps, turn signals, marker lamps, identification lamps, or safety devices such as hazardous material placards or conspicuity markings; and, (4) the device has neither the strength, rigidity nor mass to damage a vehicle, or injure a passenger in a vehicle that strikes a vehicle so equipped from the rear.

Mr. Furst concludes that FHWA has determined that the ATDynamics TrailerTail meets the conditions of 23 CFR 658.16(b)(4). He states that FHWA--

acknowledges that ATDynamics Trailer Tail was tested by an independent laboratory, KARCO Engineering, and was found to be in compliance with all elements of 23 CFR 658.16(b) (4). Therefore, in accordance with Federal regulations, the ATDynamics Trailer Tail aerodynamic device should be excluded from the length measurements for commercial motor vehicles.

KARCO Engineering determined that the ATDynamics Trailer Tail aerodynamic device Passed all of the conditions listed in the regulation, and FHWA accepts those results.

Mr. Furst also states that FHWA shared the test results with staff from the National Highway Traffic Safety Administration (NHTSA) for review. Mr. Furst states: NHTSA identified a conspicuity marking issue[and] ATDynamics has taken care of the issue in the manner NHTSA recommended.

With that background in mind, in your letter to us you ask for clarification from NHTSA with regard to the process for defining compliance with 23 CFR 658.16(b)(4) on aerodynamic devices. In other words, as we understand your letter, you ask us to confirm that use of the ATDynamics TrailerTail would not violate Federal laws administered by NHTSA.

We note that Transport Canada has also contacted us for our views on trailer tails.

NHTSAs Framework

It would be helpful in answering your question to begin with a discussion of NHTSAs authority under the National Traffic and Motor Vehicle Safety Act (49 U.S.C. Chapter 301, Vehicle Safety Act).

NHTSA is authorized under the Vehicle Safety Act to issue Federal motor vehicle safety standards (FMVSSs) applying to the manufacture and sale of new motor vehicles and items of motor vehicle equipment. Manufacturers are required to self-certify that their products conform to all applicable FMVSSs in effect on the date of manufacture. This agency does not provide approvals of new motor vehicles or of modifications of used vehicles.

NHTSA has exercised its rulemaking authority to establish a number of standards that apply to new trailers. Those standards include FMVSS No. 108, Lamps, reflective devices, and associated equipment (49 CFR 571.108), which requires trailers to have specified systems to provide adequate illumination of the roadway and to improve the conspicuity of the vehicles. Another standard applying to trailers is FMVSS No. 224, Rear impact protection (49 CFR 571.224). Standard No. 224 requires trailers to have rear impact guards to reduce the harm to occupants of light duty vehicles impacting the rear of the trailer. Each new trailer with a trailer tail sold in the U.S. must be certified by its manufacturer as complying with all applicable standards, including FMVSS No. 108 and No. 224.

After the first purchase of a vehicle for purposes other than resale, the Vehicle Safety Act limits modifications that may be made to the vehicle by commercial entities. 49 U.S.C. 30122 states:

A manufacturer, distributor, dealer, or motor vehicle repair business may not knowingly make inoperative any part of a device or element of design installed on or in a motor vehicle or motor vehicle equipment in compliance with an applicable motor vehicle safety standard prescribed under this chapter.

In the case of the motor carriers mentioned in your letter, this provision would prohibit a commercial business from installing a trailer tail on a motor carriers new or used vehicle in a manner that would negatively affect the vehicle's compliance with FMVSS No. 108 or No. 224 or any other safety standard.[1], [2]


The question posed by your letter is whether installing the ATDynamics TrailerTail on a new or used vehicle would be permitted under NHTSAs regulations.

Unfortunately, we cannot provide a sweeping answer that covers all installations of the ATDynamics TrailerTail. NHTSA assesses the compliance of new vehicles and administers the make inoperative provision of the Vehicle Safety Act concerning new or used vehicles independently from other agencies. We do not approve motor vehicles or processes undertaken by manufacturers.

A possible violation of the FMVSSs or the make inoperative provision is evaluated by NHTSA according to the facts of each particular case. Thus, NHTSA would evaluate, among other matters, the design and construction of a particular ATDynamics TrailerTail, the manner in which the trailer tail was attached, and whether the trailer tail impaired the effectiveness of the trailers lamps and other devices installed pursuant to FMVSS No. 108. We cannot prospectively and categorically affirm that all future uses of the ATDynamics device would be acceptable to this agency.

However, we recognize and appreciate the effort that has been made seeking the agencies input in exploring possible safety issues related to the ATDynamics TrailerTail. In view of those efforts, we make the following observations based on the KARCO Engineering (KE) test.

KE conducted a 35 mile per hour rear offsest crash test for ATDynamics. ATDynamics installed a TrailerTail on a 1991 Pine Trailer. The test vehicle and set-up was prepared by KE. The impacting vehicle was a 1994 Ford Econoline 350 Van. Two Hybrid III 50th percentile adult male test dummies equipped with head triaxial accelerometers to measure head injury accelerations were placed in the driver and right-front passenger seating positions. KEs report on the test states (Laboratory Test Report, Rear-Mounted Aerodynamic Device, TrailerTail mounted to a 1991 Pine Trailer, Prepared for Advanced Transit Dynamics, Inc., July 22, 2008, KARCO Engineering):

(a) Inspection of pre- and post-test photographic data showed no appreciable deformation of any structural component of the impacting vehicle attributable to the trailer tail (not including glass, plastic lenses, or trim components);

(b) The head injury criterion of neither test dummy exceeded a value of 1,000 as a result of direct contact with the trailer tail; and

(c) There was no evidence from either post-test inspection of the transfer of chalk applied to the test dummies or from still or high speed photography that the trailer tail or any resilient component of the impacting vehicle made contact with any portion of the test dummies as a result of contact of the impacting vehicle with the trailer tail.

In addition, the report indicates that the open geometry of the ATDynamics TrailerTail does not allow it to carry cargo, and that the vehicles lamps and conspicuity markings would meet FMVSS No. 108.

The test data from the KE test indicate that the ATDynamics TrailerTail did not negate the vehicles ability to meet FMVSS No. 224[3] and that the rear impact guard on the vehicle was not made inoperative by the ATDynamic TrailerTail. Thus, there is no basis for NHTSA to conclude at this time that installation of the ATDynamics TrailerTail is prohibited.

Please note that NHTSA is interested in Transport Canadas on-going work evaluating the safety and performance of trailer tails. NHTSA will evaluate the outcome of Canadas research to see if we should undertake further work on trailer tails.

If you have any other questions, please contact Deirdre Fujita of my staff at this address or by phone at (202) 366-2992.


O. Kevin Vincent

Chief Counsel


[1] The make inoperative provision applies to a manufacturer, distributor, dealer, or repair business installing a trailer tail, and not to a vehicle owner that modifies its own vehicle. However, States have the authority to regulate the operation of vehicles in their jurisdictions, and may have restrictions on the type of modifications owners may make.

[2] The Vehicle Safety Act also requires manufacturers of motor vehicles and motor vehicle equipment to ensure that their products are free of safety-related defects. A trailer tail would be considered motor vehicle equipment under the Vehicle Safety Act.

[3] That is, it appears that the trailer tail would qualify as a nonstructural protrusion under FMVSS No. 224. See S4, definition of rear extremity.